1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device which thin film transistors (TFT) are arranged in a matrix form and these thin film transistors are used as switching elements.
2. Description of the Related Art
An active matrix type TFT (Thin Film Transistor: hereinafter abbreviated as “TFT”) liquid crystal display device in which TFTs are formed in a matrix arrangement on a glass substrate and these TFTs are used as switching elements has been developed as a high-quality flat-face display. In a twisted nematic (hereinafter abbreviated as “TN”) type active matrix crystal display device which has been hitherto widely used, transparent electrodes which are formed on two glass substrates so as to confront each other are used as electrodes for driving a liquid crystal layer. By applying a voltage to liquid crystal molecules which are arranged in parallel to the substrate surface under non-voltage application state (i.e., “white” display state), the direction of the orientation vector of the liquid crystal molecules varies from the “white” display state to the direction of the electric field in accordance with the applied voltage, whereby the “white” display state is gradually varied to a “black” display state.
However, the inherent behavior of the liquid crystal voltages under voltage-applied state causes a problem that the angle of visibility of the TN type liquid crystal display device is small. The problem that the angle of visibility is small is particularly remarkable in the rise-up direction of the liquid crystal molecules under a half tone display state.
A technique as disclosed Japanese Laid-open Patent Application No. Hei-4-261522 or Japanese Laid-open Patent Application No. Hei-6-43461 has been proposed as a method of improving the angle-of-visibility characteristic of the liquid crystal display device. According to these techniques, a liquid crystal cell in which liquid crystal molecules are homeotropically oriented is created, and it is sandwiched between two polarizing plates arranged so that the polarization axes thereof are perpendicular to each other. As shown in the drawings of the above publications, a slant electric field is generated in each pixel by using a common electrode having an opening portion to make two or more crystal liquid domains in each pixel, thereby enhancing the angle-of-visibility characteristic. In the Japanese Laid-open Patent Application No. Hei-4-261522, the slant direction of the liquid crystal molecules when the voltage is applied is particularly controlled to achieve high contrast.
Further, as disclosed in Japanese Laid-open Patent Application No. Hei-6-43461, an optical compensator is used to enhance the angle-of-visibility characteristic for black, as occasion demands. Further, in Japanese Laid-open Patent Application No. Hei-6-43461, for not only a homeotropically-oriented type of liquid crystal cell, but also a TN-oriented type liquid crystal cell, each pixel is divide into two or more domains by using slant electric field, thereby enhancing the angle-of-visibility characteristic.
Japanese Patent No. Hei-5-505247 proposes an IPS (In-Plane-Switching) type liquid crystal display device in which two electrodes are formed on one substrate and a voltage is applied across these two electrodes to generate electric field in parallel to the substrate in order to rotate the liquid crystal molecules while keeping the molecules in parallel to the substrate. According to this system, the major axis of each liquid crystal molecule is prevented from rising up with respect to the substrate when the voltage is applied. Therefore, the variation of the birefringence of the liquid crystal molecules when the direction of the visual angle is varied is small, and thus the angle of visibility is large.
An IPS type active matrix liquid crystal device in which both of two electrodes are provided on one of substrates as described above will be described hereunder. The IPS type TFT liquid crystal display device is constructed as shown in FIGS. 12A and 12B. FIG. 12A is a cross-sectional view taken along A-A′ line of a plan view of FIG. 12B.
First, a gate electrode 1202 and a common electrode 1203 are formed of Cr on a glass substrate 1201, and then a gate insulating film 1204 of silicon nitride is formed so as to cover these electrodes 1202 and 1203. Then, a semiconductor film 1205 of amorphous silicon is formed through a gate insulating film 1204 on the gate electrode 1202, and it functions as an active layer of transistors. A drain electrode 1206 and a source electrode 1207 are formed of molybdenum so as to be superposed over a part of the pattern of the semiconductor film 1205, and a protection film 1208 of silicon nitride is formed so as to cover all the above elements.
As shown in FIG. 12B, a one-pixel area is disposed between the source electrode 1207 and the drawn-out common electrode 1203. Thereafter, an orientation film ORI 1 is formed on the surface of an active matrix substrate in which a plurality of unit pixels thus constructed are arranged in a matrix form. The surface of the orientation film IRI1 is subjected to a rubbing treatment.
Further, a color filter layer 1232 is formed on a counter substrate 1231 of glass so as to be partitioned by light shielding portions 1233, and a protection film 1234 is formed on these elements. An orientation film ORI2 is also formed on the surface of the protection film 1234, and the surface of the orientation film ORI2 is also subjected to the rubbing treatment.
The glass substrate 1201 and the counter substrate 1231 are disposed so that the orientation film ORI1 and the orientation film ORI2 are confronted to each other, and liquid crystal composition 1240 is disposed between the orientation films ORI1 and ORI2. Further, a polarizing plate 1251 is formed on each of the outer surfaces of the glass substrate 1201 and the counter substrate 1231. Each of the light shield portions 1233 through which the color filter layer 1232 is partitioned is partially disposed on a thin film transistor formed of the semiconductor layer 1205.
In the active matrix type liquid crystal display device thus constructed, when no electric field is applied to the liquid crystal composition 1240, liquid crystal molecules 1241a are kept to be substantially parallel to the extending direction of the electrodes, and homogeneously oriented. That is, the liquid crystal molecules 1241a are orientated so that the intersecting angle between the direction of the major axis (optical axis) of the liquid crystal molecules 1241a and the direction of the electric field formed between the source electrode 1207 and the drawn-out common electrode 1203 is set to a value in the range which is above 45° and less than 90°. The glass substrate 1201 and the counter substrate 1231 arranged so as to confront each other are disposed in parallel to the orientation direction of the liquid crystal molecules 1241a. The permittivity anisotropy of the liquid crystal molecules 1241a is set to a positive value.
Here, when a voltage is applied to the gate electrode 1202 to switch on the thin film transistor (TFT), a voltage is applied to the source electrode 1207 to induce electric field between the source electrode 1207 and the common electrode 1203 disposed so as to confront the source electrode 1207. The liquid crystal molecules 1241a are orientationally turned to liquid crystal molecules 1241b. The liquid crystal molecules 1241b are kept to be substantially parallel to the direction of the electric field generated between the source electrode 1207 and the common electrode 1203 disposed so as to confront the source electrode 1207.
By setting the polarization transmission axis of the polarizing plate 1251 at a predetermined angle, the transmittance of light can be varied by the movement of the liquid crystal molecules as described above.
As described above, with the IPS type active matrix liquid crystal display device, the contrast can be given without any transparent electrode.
In the IPS type TFT liquid crystal display device, the major axis of the liquid crystal molecules is substantially parallel to the flat surface of the substrate, and it does not rise up even when a voltage is applied. Therefore, variation in brightness when a viewing direction is varied is little, and thus the visual characteristic is greatly enhanced.
Further, a paper (Journal of Applied Physics, Vol. 45, No. 12 (1974) 5466) or Japanese Laid-open Patent Application No. Hei-10-186351 discloses such a system that liquid crystal molecules having positive permittivity anisotropy are homeotropically oriented perpendicularly to the substrate and these molecules are felled and put in parallel to the substrate by the electric field directing in parallel to the substrate, in addition to an IPS mode. At this time, the liquid crystal molecules which are homeotropically oriented due to the direction of the electric field are divided into two or more areas which are different in the slant direction of the liquid crystal molecules.
However, in the IPS system, the color filter layer is disposed between the liquid crystal layer and the counter substrate, and thus the electric field which will be formed when potential is applied between the source electrode and the drawn-out common electrode adversely affects the color filter layer and degrades the display characteristic of the active matrix type liquid crystal display device. That is, the pigments constituting the color filter layer contain sodium ions, etc., and thus when electric field is applied to the color filter layer, charges are trapped there and the color filter layer is charged up. When the color filter layer charges up, undesired electric field is applied to the liquid crystal molecules below the charge-up area of the color filter layer at all times, so that the display characteristic is adversely effected.